The present invention relates generally to magnetic resonance imaging (MRI) systems and specifically to MRI systems using permanent magnets.
Magnetic resonance imaging (MRI) systems are widely used in medical community as a diagnostic tool for imaging the tissue and organ structures of a patient. MRI systems establish a primary magnetic field, and a series of gradient fields that influence gyro magnetic materials in the object to be imaged. During imaging, gradient fields are pulsed in accordance with predetermined imaging protocols, and a radio frequency field causes motion of molecules of the gyro magnetic materials. Signals resulting from realignment of the molecules are then detected and processed to reconstruct useful images of the object. MRI magnet designs include closed magnets and open magnets.
Closed magnets typically have a single, tubular-shaped bore in which the object may be positioned for imaging. Open magnet designs, including “C” or “U” shaped magnets, typically employ two magnet assemblies separated by a space from one another, with the space between the magnet assemblies defining an imaging volume. The object to be imaged, such as a patient, is positioned in the imaging volume for imaging. In open MRI systems, the space between the magnet assemblies aids certain patients in remaining comfortable during examinations and also allows for access by medical personnel for surgery or any other medical procedure during magnetic resonance imaging.
Image quality in MRI systems depends on the stability of the main magnetic field. In MRI systems employing permanent magnets, such as open MRI systems, the main magnetic field may fluctuate in response to temperature changes in the permanent magnet. Therefore, stability of the main magnetic field in these MRI systems depends on maintaining thermal stability of the permanent magnets. In order to maintain thermal stability, the permanent magnets are generally maintained at a set temperature, which is typically higher than the ambient room temperature. For example the permanent magnets may be maintained at a temperature of 30 degrees Celsius when the ambient room temperature is around 22 degrees Celsius.
Because electrical heaters, such as resistive heaters generate magnetic fields, which can affect the stability of the main magnetic field, it is generally infeasible to directly heat the permanent magnets of such an open system using electrical techniques. Instead, temperature control of the permanent magnets in a permanent magnet MRI system is typically achieved by controlling the temperature of the support structure, known as the yoke, which holds the permanent magnets.
Such indirect temperature control may be undesirable for a variety of reasons, however. For example, the mass of the yoke may require substantial heating to achieve the desired temperature increase in the attached permanent magnet. Furthermore, such indirect control may make precise control of the permanent magnet temperature difficult. In particular, the delay between a temperature change in the permanent magnet after heat is applied to the yoke may result in an undesirable lag time during which the main magnetic field is not stable.
Furthermore, as discussed above, MRI systems also include a gradient coil to generate the gradient field. The gradient coil is typically close to the permanent magnets in open MRI systems. During imaging, gradient currents passing through the gradient coil may increase the temperature of the permanent magnets above, resulting in additional thermal instability and, hence, main magnetic field instability. Under these circumstances, use of the resistive heaters discussed above for maintaining thermal stability is ineffective since the permanent magnet is already hotter than desired.
Thus there exists a need for an effective method and system for maintaining thermal stability of the permanent magnets in the MRI systems.